Hello Adrian,
I tried to use -U without "su"- launched directly by root: same behaviour.
Finally I reverted my script to use standard backup (pg_start_backup; rsync; pg_stop_backup)- this works- the only downside is possible collisions with on-line backup/synchronizaiton of other two nodes on master node...
Back to the pg_basebackup issue: it is clear to me that this is an issue of environment which launched pg_basebackup.
Possibly either some privileges or some kernel parameters/limits. Who knows?
Summary: clusterlab's crm_mon launched a shell script starting pg_basebackup which fails to do some its work (pg_basebackup: could not wait for child process: No child processes)- probably due to some failing system call.
How can I report to clusterlabs: What system call fails in pg_basebackup?
Best Regards, Andrej